The present invention generally relates to a heater arrangement for a furnace and, more specifically, to a heater arrangement for a crystal growth furnace suitable for producing a high volume of crystals.
Furnaces for the production of crystals, such as single crystals of calcium fluoride, typically have a crucible which is loaded with a seed and/or starting material. A heater (or heaters) is arranged about the crucible to produce a temperature gradient to grow the crystals in the crucible. Growth is obtained by varying power to the heater according to an established power-temperature relationship to obtain the desired thermal environment.
The thermal gradient obtained is critical to growing a single crystal rather than a polycrystalline structures. Additionally, the quality of a single crystal is believed to be primarily affected by the applied thermal gradient. Present furnaces for the production of macrocrystals, therefore, have elaborate and complex heaters and/or controllers for controlling the heaters to obtain the desired thermal environment. These complex devices are expensive to produce and complicated to operate and maintain.
Further, high quality calcium fluoride crystals are of great need in the semiconductor manufacturing industry. Commercial success in growing large, monocrystalline ingots from calcium fluoride, has been limited to ingots have a diameter of 15 inches or less.
In high volume production of crystals it is important to obtain the desired thermal environment and is also important to get consistent temperature environments. In addition, large diameter single-crystal growth requires increased precision and consistency in control of temperature environment. Accordingly, there is a need in the art for a crystal growth furnace which is simple to produce and operate, produces desired thermal environments for growing crystals, and produces consistent thermal environments for growing a high volume of crystals and for growing crystals of a large diameter.
The present invention provides a furnace for growing crystals which overcomes at least some of the above-noted problems of the prior art. According to the present invention, a heater arrangement for a crystal growth furnace includes a plurality of individual growth stations each having a crucible. The crystal growth furnace also includes a first heater matrix having at least two resistance heaters electrically connected in series or parallel. Each of the individual growth stations has at least one of the resistance heaters of the first heater matrix associated therewith and located near the crucible. By connecting the resistance heaters of separate growth stations in this manner, the temperatures produced by the resistance heaters in the separate growth stations are fixed at the same temperature for a given power level when the resistance heaters are connected to a single power source.
According to another aspect of the present invention, a heater arrangement for growing crystals includes a plurality of individual growth stations each having a crucible. The heater arrangement also includes a first heater matrix and a second heater matrix separate from the first heater matrix. Each heater matrix preferably includes at least two legs electrically connected in parallel with each of the legs having at least two resistance heaters electrically connected in series. Each of the individual growth stations has at least one of the resistance heaters of the first heater matrix and at least one of the resistance heaters of the second heater matrix associated therewith. By having two separate heater matrices, the temperatures produced by the resistance heaters in a large quantity of separate. growth stations can be fixed at the same temperature for a given power level yet the temperature gradient formed in each of the growth stations can be varied when each heater matrix is connected to a separate power source. Preferably, the resistance heaters within the first heater matrix are located above the crucibles and provide a homogeneous temperature across the top of the crucibles and the resistance heaters within the second heater matrix are located below the crucibles and provide a temperature gradient across the bottom of the crucibles.